regvprobs
Now explain this ?
the pos reg works fine 14.89v, changed the neg reg twice and still the same happens..21.7v
Im now thinking that it could be the transformer/rectifier, as ive got a dual secondary and iv'e tied two legs to ground.
Just so you know im a qualified electrician, so you can leave out the basic's if you get my drift.
What i may do but three week old Harry is taking our my sunday,
is to use two rectifer's instead of one.
Now explain this ?
the pos reg works fine 14.89v, changed the neg reg twice and still the same happens..21.7v
Im now thinking that it could be the transformer/rectifier, as ive got a dual secondary and iv'e tied two legs to ground.
Just so you know im a qualified electrician, so you can leave out the basic's if you get my drift.
What i may do but three week old Harry is taking our my sunday,
is to use two rectifer's instead of one.
The 40V rating of the capacitor has nothing with this to do but I'm not explaining why since you told us to leave out the basics. 
If you are using the TO220 version of the regulators, make sure the 7915 mounting tab is not grounded, it's connected to the output (middle pin).
Other things to try, connect small 1 to 10 uF capacitors as close as possible to the input and output of the 7915. Negative regulators are more prone to oscillate.
If you are using the TO220 version of the regulators, make sure the 7915 mounting tab is not grounded, it's connected to the output (middle pin).
Other things to try, connect small 1 to 10 uF capacitors as close as possible to the input and output of the 7915. Negative regulators are more prone to oscillate.
faulty tx
Thanks for the images, iv'e got them written down in one of my books anyway.
Just split the tx secondaries and used two rectifers, one pos and one neg. Now getting 40v per rail under no load condition , bear in mind that its an 18v tx and one would expect around 22v before the rectifiers (output *1.414) if my memory is correct ?
And around what after the rectifiers ? 40v seems abit to high !!
Thanks for the images, iv'e got them written down in one of my books anyway.
Just split the tx secondaries and used two rectifers, one pos and one neg. Now getting 40v per rail under no load condition , bear in mind that its an 18v tx and one would expect around 22v before the rectifiers (output *1.414) if my memory is correct ?
And around what after the rectifiers ? 40v seems abit to high !!
Is this the catch?
According to the ST datasheet:
ST L7915CV
It seems that load regulation is only specified down to 5mA.
According to the ST datasheet:
ST L7915CV
It seems that load regulation is only specified down to 5mA.
Re: Is this the catch?
It is the same in the National datasheets, so it could be the catch.
Although not the same circuit, I also checked LM337. It also has
a similar catch and in addition specifies a minimum load current
to maintain regulation, in this case you may have to draw as much
as 10mA minimum.
Generally, I think it is wise to always have some load on a regulator
unless one is absolutely certain it is not required.
dhaen said:
It is the same in the National datasheets, so it could be the catch.
Although not the same circuit, I also checked LM337. It also has
a similar catch and in addition specifies a minimum load current
to maintain regulation, in this case you may have to draw as much
as 10mA minimum.
Generally, I think it is wise to always have some load on a regulator
unless one is absolutely certain it is not required.
reg probs
Hi guy's
Im not sure what current this phono stage requires, i know it wont be to much , this is what this reg board if for. So should i use a loading resistor on the output of the reg? (tried a 1k5 @ 2w)But this made no difference.
Im being a cheapskate here by the way, as this kit come's with a power supply and i thought it would be easier and cheaper to build my own. The only difference between mine and there's is a pcb! and there's has three lots of reg's on each rail for different applications, and all being 15v regs ???
Hi guy's
Im not sure what current this phono stage requires, i know it wont be to much , this is what this reg board if for. So should i use a loading resistor on the output of the reg? (tried a 1k5 @ 2w)But this made no difference.
Im being a cheapskate here by the way, as this kit come's with a power supply and i thought it would be easier and cheaper to build my own. The only difference between mine and there's is a pcb! and there's has three lots of reg's on each rail for different applications, and all being 15v regs ???
Well, your 1k5 (100K ohms, right?) resistor is only drawing 150 microamps at 15V.. so that isn't quite up to the 5-10mA the regulator needs to regulate.
Try a 3000 to 1500 ohm resistor for that current range.. Assuming you didn't get caught out by the different pinout of the neg regulator, and it's smoked, then you should be working..
On the really high rectified voltage, it's typical for transformer AC voltage to be quoted at a certain load.. transformers tend not to regulate very well (depends, and smaller are worse) so you might be starting rather high, the rectifier will give you the peak if has next to no load.. so you can be pretty high from expected.
Check your wiring with the transformer though.. You don't want any higher input voltage than you need, for a couple of reasons: You dont want to exceed the max rating, as you've already picked up on, and you will have to burn more power in the regulator while dropping the higher voltage. Remember, every volt you drop equals the current times that voltage of heat in the regulator!
So, if you're actually dropping 40V to 15V, and your amp or whatever takes 100mA, that's 25 volts drop times .1A = 2.5 watts, and you're going to have a really hot regulator! As opposed to say, 18v, and you only have .3W to dissapate, maybe you dont even need a heat sink..
I'd try to arrange the transformer/rectifier setup to give me as close to (but remember the min dropout voltage, you need a volt or two headroom) my regulated voltage as I can. Using a resistor that simulates the total load of the final circuit so I'd know what the "real" voltages will be under load.
Let us know how it turns out!
Jon
Try a 3000 to 1500 ohm resistor for that current range.. Assuming you didn't get caught out by the different pinout of the neg regulator, and it's smoked, then you should be working..
On the really high rectified voltage, it's typical for transformer AC voltage to be quoted at a certain load.. transformers tend not to regulate very well (depends, and smaller are worse) so you might be starting rather high, the rectifier will give you the peak if has next to no load.. so you can be pretty high from expected.
Check your wiring with the transformer though.. You don't want any higher input voltage than you need, for a couple of reasons: You dont want to exceed the max rating, as you've already picked up on, and you will have to burn more power in the regulator while dropping the higher voltage. Remember, every volt you drop equals the current times that voltage of heat in the regulator!
So, if you're actually dropping 40V to 15V, and your amp or whatever takes 100mA, that's 25 volts drop times .1A = 2.5 watts, and you're going to have a really hot regulator! As opposed to say, 18v, and you only have .3W to dissapate, maybe you dont even need a heat sink..
I'd try to arrange the transformer/rectifier setup to give me as close to (but remember the min dropout voltage, you need a volt or two headroom) my regulated voltage as I can. Using a resistor that simulates the total load of the final circuit so I'd know what the "real" voltages will be under load.
Let us know how it turns out!
Jon
Voltages
Thanks for your reply Jon.
I will try that method at the weekend , don't get enough time in the week, as our new born take's up most of my evening !!!
I used a 1500ohm to load the reg, but iv'e got some more so i will series a few. Put the caps be in the circuit and found that i was picking up the ripple voltage...duurrr silly me !!! Back to 30v now
So i'll let you anyway....i wished id spent more time looking and listening to the teachers at school, than the windows ????
Thanks for your reply Jon.
I will try that method at the weekend , don't get enough time in the week, as our new born take's up most of my evening !!!
I used a 1500ohm to load the reg, but iv'e got some more so i will series a few. Put the caps be in the circuit and found that i was picking up the ripple voltage...duurrr silly me !!! Back to 30v now
So i'll let you anyway....i wished id spent more time looking and listening to the teachers at school, than the windows ????
OK, sorry for missing the nomenclature there, thought it meant 1.5 x 10E5..
If you gave it 10ma load and it didn't start regulating, something's wrong and more probably won't change things. Regulator might be bad..
How are you wiring the transformer, diodes and ground again? Just re-read the part about only one diode (total?? each side?) and am wondering.
If you gave it 10ma load and it didn't start regulating, something's wrong and more probably won't change things. Regulator might be bad..
How are you wiring the transformer, diodes and ground again? Just re-read the part about only one diode (total?? each side?) and am wondering.
Prob solved
Hi Jon,
I found the fault.
Hallelujah.
After all that ****ing in the wind, it was a dry joint on the ground of the test point...duurr Now got 14.88v with a 1500ohm load.
Thanks to everyone for the help on this, its made my day and now im off to bed.
Does anyone out there have two 470uf @25v caps that they could possibly give to me?. They must be less than 10mm in dia, tried maplin but to no avail. Rs only sell them in 5s and Farnell won't do orders less than £10.
Thanks Guys.
Pete.
Hi Jon,
I found the fault.
Hallelujah.
After all that ****ing in the wind, it was a dry joint on the ground of the test point...duurr Now got 14.88v with a 1500ohm load.
Thanks to everyone for the help on this, its made my day and now im off to bed.
Does anyone out there have two 470uf @25v caps that they could possibly give to me?. They must be less than 10mm in dia, tried maplin but to no avail. Rs only sell them in 5s and Farnell won't do orders less than £10.
Thanks Guys.
Pete.
Congrats!
Yep, the ol' Sherlock Holmes thing.. "If you have eliminated all the other possibilites... "
Many times I've gotten to a head-scratching point, walk away for a time.. to come back and not even BELIEVE the very obvious thing I wasn't seeing for so long.
I've learned it's important to not stare too long at a problem, and be able to "adjust my eyes" or "change my perception" by leaving it for a while, then coming back.
Probably want to leave it in there, or one on both regulators, unless heating is a problem. Curious what your unregulated voltage ended up to be, with the load and proper ground...
Yep, the ol' Sherlock Holmes thing.. "If you have eliminated all the other possibilites... "
Many times I've gotten to a head-scratching point, walk away for a time.. to come back and not even BELIEVE the very obvious thing I wasn't seeing for so long.
I've learned it's important to not stare too long at a problem, and be able to "adjust my eyes" or "change my perception" by leaving it for a while, then coming back.
Probably want to leave it in there, or one on both regulators, unless heating is a problem. Curious what your unregulated voltage ended up to be, with the load and proper ground...
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